1. Field of the Invention
The present invention relates to a catalyst, a membrane electrode assembly and a fuel cell provided with the catalyst and a method of manufacturing the catalyst.
2. Description of the Related Art
Polymer electrolyte fuel cells and particularly, polymer electrolyte fuel cells using a methanol aqueous solution as a fuel can work at low temperatures and also be made small and light. Studies have therefore been pursued in recent years concerning using these cells as power sources for mobile devices. However, the performances of fuel cells is still not enough for commercialization. Fuel cells convert chemical energy into electrical energy by the catalytic reaction of the electrode. A highly active catalyst is essential to develop a high performance fuel cell.
As the anode catalyst of a fuel cell, PtRu is usually used. The voltage loss of the PtRu catalyst is about 0.3 V which accounts for about 25% of the voltage 1.21 V theoretically allowed to be generated. Therefore, there is a demand for an anode catalyst having an activity such as methanol oxidation activity exceeding that of PtRu. In order to enhance methanol oxidation activity, various studies including adding other elements to PtRu have been made and reported.
For example, the publication of U.S. Pat. No. 3,506,494 refers to the effect obtained by adding 10 types of metals such as tungsten, tantalum and niobium to PtRu. It should be noted that catalytic activity depends on synthetic process significantly even if the catalyst has the same composition, because the catalytic reaction occurs on the surface of each of nano-size catalyst particles, and several atomic layers in the surface of the catalyst particle largely affect catalytic activity. For instance, JP-A 2005-259557 (KOKAI) relates to a method of manufacturing an anode catalyst by adding the IV to VI group metal to Pt and Ru by a dipping method. JP-A 2005-259557 (KOKAI) reports that the methanol activity is largely varied by the dipping sequence of the adding metals, Pt and Ru. As to the ratio of Pt, Ru to the IV to VI group metals, it is only described that Pt:Ru:Metals to be added=317.7:82.3:100.
It is considered that there is still a large possibility to develop catalyst having high activity over PtRu by controlling the synthetic process to synthesize catalyst particles having a nano-structure which has never existed. Solution reaction methods such as a dipping method have been generally used in catalyst synthesis so far. However, it is difficult for solution reaction methods to control the structure and surface of catalyst particles made of elements which are unlikely to be reduced or alloyed.
It is advantageous to each of a sputtering method and a deposition method in view of synthesizing the catalyst particles made of elements which are unlikely to be reduced or alloyed. However, much study has not yet been made concerning the effects of the type of elements, composition of the catalyst particles, substrate material, substrate temperature and the like on the sputtering method and deposition method.
When catalyst particles are nanoparticles, the state of electrons on the surface of the catalyst particles and the nano-structure of the catalyst particles tend to be largely dependent on the type and amount of elements to be added to the catalyst particles. It is considered that in order to obtain catalyst particles having high activity and high stability, it is necessary to make appropriate the type of elements to be added to the catalyst particles, the amount of elements to be added and the combination of elements.
JP-A 2004-281177 (KOKAI) reports the effect of Sn and W added to a Pt—Ru alloy when a catalyst is formed on a substrate made of a gold foil or Si by a sputtering method. However, it cannot be said that a catalyst having sufficient methanol oxidation activity is established. There is no description as to the effect of the addition of elements other than Sn and W in JP-A 2004-281177 (KOKAI). As to the effect obtained when Sn is added, the effect obtained when the amount of Sn in a catalyst layer is 25% is reported.
PCT National Publication No. 2005-532670 discloses a catalyst useful for the electrolytic oxidation of a fuel in a fuel cell using a proton exchange membrane. The catalyst is prepared by chemically activating substantially semicrystalline PtXaAlb deposited on a base material. In the above composition formula, an element selected from the group consisting of Ru, Rh, Me, W, V, Hf, Zr, Nb and Co is used as X on the condition that when “a” is 1 and b is 8, X is selected from W, V, Hf, Zr, Nb and Co. “a” is 0.001 at the least and b is 0.85(1+ a) at the least.
JP-A 2005-334685 (KOKAI) describes that second microparticles with first microparticles stuck to a surface thereof are used as catalyst particles. In this case, a thin film is formed on the surface of the first microparticles by sputtering.
The publication of U.S. Pat. No. 6,171,721B1 discloses that a catalyst is formed on the surface of an electrolyte membrane by sputter-depositing.
JP-A 2004-281177 (KOKAI) discloses that an alloy made from at least one element selected from tungsten, tin, molybdenum, copper, gold, manganese and vanadium, platinum and ruthenium is used as a methanol oxidation electrode catalyst.
In the meantime, a catalyst having a composition Pt—Ru—Ni—Zr is disclosed in S. R. Narayanan, Jay Whitacre DOE Hydrogen Program FY2004, Progress Report p 610.
As shown by the above documents, it has been proposed to use a sputtering process and to use many elements other than Pt and Ru for the preparation of a catalyst. However, investigation on catalyst compositions is not enough and it cannot be said that catalysts having sufficient methanol oxidation activity have been established.